The conventional passive power factor correction circuit is constituted by some passive elements such as inductors, capacitors, diodes and the likes. By changing paths which allow the input current flowing into the filter capacitor and time, the waveform and phase of input current are ameliorated, the harmonic is suppressed and the power factor is improved. Although the power factor obtained from the passive power factor correction circuit is less than the active power factor correction circuit, the passive power factor correction circuit can still make the circuit power factor value increased to 0.7˜0.9, current harmonic distortion total amount declined to below 50%. In addition, the passive power factor correction circuit further has some advantages, such as no active switch element, simple circuit structure, low cost and no electromagnetic interference problem generated form the active switch, so now it is still widely used in medium and small power electronic equipments. However, the operation point designed by the passive power factor correction circuit is easily affected by the input voltage, input voltage frequency and output power, so as to affect the result of power factor correction.
The conventional AC/DC converter works under the difference between the input voltage and the current phase so as to lower the power factor and seriously distort current total harmonic. Furthermore, only if the input voltage is higher than the capacitor voltage, the output capacitor is charged by the input power so as to decrease the charge time of the capacitor, and the turn-on time of the rectified diode gets shorter with the charge time and the peak value of the conductive current increases, causing that the power factor is decreased and the input current waveform is distorted. The equipments with lower power factor not only waste energy but also increase unnecessary reactive power required by the electricity company. Therefore, how to improve the power factor has become one of the important topics for power electronics.
The conventional power factor correction circuit can be divided into the active power factor correction circuit and the passive power factor correction circuit according to whether it has active switch elements. FIG. 1 shows a bridge rectified circuit with single phase being one of conventional passive power factor correction circuits. Owing to the output filter capacitor Cdc existence, it causes that the input current contains large amount of harmonic distortion so as to lower the power factor and seriously distort current total harmonic distortion. Moreover, the inductance element of the traditional passive correcting circuit is mainly composed of silicon steel sheet and connected the bridge rectifier input end in tandem, as shown in the single-phase circuit of FIG. 2, or the inductance element also can combine with a capacitor to become a LC filter or CLC (π type) filter. Since the inductor composed of silicon steel sheet is bulky and heavy, and the highest power factor of the circuit is only up to around 75%. Additionally, size of the passive power factor correction circuit will be expanded while the output power increase or the rated input voltage decrease. Although the bridge rectified circuit of the conventional passive power factor correction circuit has simple and durable advantages, its poor performance does not meet nowadays application needs.
The main advantages of the conventional active power factor correction circuit include the power factor being up to 0.99, the total amount of electric current harmonic distortion being less than 10%, as shown in the single-phase circuit of FIG. 3. Moreover, the advantages further include the input voltage with wider range, the stable output voltage, the small size of magnetic element, and not be affected by the output power variation. However, the active power factor correction circuit requires extra control circuit and the price, electromagnetic noise and robustness thereof are nowhere near as good as the passive power factor correction circuit.
The architecture of conventional power factor correction circuit with low frequency is similar as the conventional active power factor correction circuit, besides the active switch elements works at low frequency switching. Basic operation principles of the conventional power factor correction circuit with low frequency is to use two times of the commercial frequency as the switch frequency of the active switch element so as to reduce the high frequency electromagnetic interference, and it obtains the perfectly power factor value according to the variation of the switch-off and turn-on time, as shown in single phase circuit of FIG. 4. What is more, the advantages of the power factor correction circuit with low frequency include high efficiency, simple controlling, do not need high-power element and no high-frequency electromagnetic interference. For the household appliance system which relies on frequency convector as a main part, such as refrigerators, air conditioning etc., the power factor correction circuit with low frequency can effectively improve the power factor and reduce the total amount of current harmonic distortion.